title: Synthesis and characterization of MnCrO4, a new mixed-valence antiferromagnet.
abstract: A new orthorhombic phase, MnCrO4, isostructural with MCrO4 (M = Mg, Co, Ni, Cu, Cd) was prepared by evaporation of an aqueous solution, (NH4)2Cr2O7 + 2 Mn(NO3)2, followed by calcination at 400 °C. It is characterized by redox titration, Rietveld analysis of the X-ray diffraction pattern, Cr K edge and Mn K edge XANES, ESR, magnetic susceptibility, specific heat and resistivity measurements. In contrast to the high-pressure MnCrO4 phase where both cations are octahedral, the new phase contains Cr in a tetrahedral environment suggesting the charge balance Mn(2+)Cr(6+)O4. However, the positions of both X-ray absorption K edges, the bond lengths and the ESR data suggest the occurrence of some mixed-valence character in which the mean oxidation state of Mn is higher than 2 and that of Cr is lower than 6. Both the magnetic susceptibility and the specific heat data indicate an onset of a three-dimensional antiferromagnetic order at TN ≈ 42 K, which was confirmed also by calculating the spin exchange interactions on the basis of first principles density functional calculations. Dynamic magnetic studies (ESR) corroborate this scenario and indicate appreciable short-range correlations at temperatures far above TN. MnCrO4 is a semiconductor with activation energy of 0.27 eV; it loses oxygen on heating above 400 °C to form first Cr2O3 plus Mn3O4 and then Mn1.5Cr1.5O4 spinel.
authors.affiliation: Chemistry Faculty, Southern Federal University , 7 ul. Zorge, Rostov-na-Donu, 344090 Russia.
authors.forename: Vladimir B
authors.initials: VB
authors.lastname: Nalbandyan
authors.forename: Elena A
authors.initials: EA
authors.lastname: Zvereva
authors.forename: Galina E
authors.initials: GE
authors.lastname: Yalovega
authors.forename: Igor L
authors.initials: IL
authors.lastname: Shukaev
authors.forename: Anastasiya P
authors.initials: AP
authors.lastname: Ryzhakova
authors.forename: Alexander A
authors.initials: AA
authors.lastname: Guda
authors.forename: Alessandro
authors.initials: A
authors.lastname: Stroppa
authors.forename: Silvia
authors.initials: S
authors.lastname: Picozzi
authors.forename: Alexander N
authors.initials: AN
authors.lastname: Vasiliev
authors.forename: Myung-Hwan
authors.initials: MH
authors.lastname: Whangbo